This invention relates to a process for improving the mechanical properties of thermoplastic polymers by reacting polyisocyanates with low molecular weight chain lengthening agents in the molten thermoplast inside a screw extruder. Hard segments are thereby formed in the thermoplast and a thermoplastic mixed polymer with improved hardness and strength is obtained.
Processes for the continuous production of thermoplastic polyurethanes in double shaft screw extruders with self-cleaning screws by the reaction of relatively high molecular weight polyhydroxyl compounds, polyisocyanates and chain lengthening agents have been disclosed in German Offenlegungsschriften Nos. 2,302,564 (U.S. Pat. No. 3,963,679); 2,423,764; and 2,549,372. In these processes, the reaction mixture must be mixed vigorously inside the screw extruder with the aid of kneading elements at a stage in which the melt still has a low viscosity (about 20 to 70 Pa.s) so as to avoid inhomogeneity in the end product. According to one variation of these processes, aggregates such as thermoplasts may be mixed with the product in the screw extruder during or after the reaction but there is no indication in these publications that the mechanical properties of thermoplasts could be improved by the controlled incorporation of polyurethane hard segments.
U.S. Pat. No. 3,049,505 and German Offenlegungsschrift No. 1,570,073 relate to mixtures of thermoplastic polyurethanes and graft copolymers which are distinguished by their high tensile strength at break. These publications, however, give no indication that such mixtures could be prepared in a screw reactor or that a hard polyurethane could be produced in the presence of the graft copolymer.
German Offenlegungsschrift No. 2,011,508 relates to a process for the production of very hard thermoplastic polyurethane elastomers from softer thermoplastic polyurethanes by mixing the soft polyurethanes with from 5 to 50% by weight of "hard components" as fillers. These "hard components" are reaction products of diisocyanates with low molecular weight glycols. No mention is made in the publication, however, of the use of screw reactors. Since the finished hard components are added as fillers in the process according to German Offenlegungsschrift No. 2,011,508, the end products obtained are relatively inhomogeneous and have unsatisfactory strength properties. By contrast, in the process according to the present invention, the hard component is formed inside a screw reactor in the presence of the previously prepared softer thermoplast. This results in completely homogeneous products of high strength.